Electrically driven oxygen separation devices employ one or more composite membrane elements that have cathode and anode electrodes situated on opposite sides of an electrolyte to apply an electric potential difference to the electrolyte. The electrolyte is a ceramic material that at elevated temperature and upon the application of the potential difference is capable of oxygen ion transport when heated to an elevated operational temperature. When the electric potential difference is applied to the electrodes and the cathode electrode is contacted with an oxygen containing feed stream, oxygen ion transport will be induced through the electrolyte to separate the oxygen from the oxygen containing feed stream to produce molecular oxygen at the anode collector. Such oxygen separation devices can be used to purify the feed stream or to produce an oxygen product when the feed stream contains sufficient oxygen in case of air. Typically, the electrically driven oxygen separator has an electrically heated enclosure that contains the composite membrane elements to heat such membrane elements up to the operational temperature at which oxygen ion transport is possible through the electrolyte.
Although there are various forms of such electrically driven oxygen separation devices, the oxygen separation elements used in such separation devices all employ an electrolyte that is at least primarily an oxygen ion conductor such as gadolinium doped ceria or more exclusively an oxygen ion conductor such as Scandium and/or yytrium stabilized zironia and electrically conductive electrodes that can be an electrically conductive perovskite covered by a conductive current collector fabricated from silver. The composite membrane elements can take a variety of forms, such as a single tube, a series of tubes, flat plates and plate-like structures having integrally molded tubular projections. For example, as illustrated in US Patent Appln. Serial No. 2010/116133, an electrically driven oxygen separation device is illustrated that utilizes bundles of tubular elements that are connected by manifolds that are in turn enclosed in an electrically heated enclosure. Each tubular element is provided with a cathode layer, an anode layer and an electrolyte layer. Additionally, two current collector layers are located adjacent the anode and cathode layers where the electric potential difference is actually applied to the anode and cathode. Each of the cathode and anode layers is formed from (La0.8Sr0.2)0.98MnO3-δ. The electrolyte layer is 6 mol % scandium doped zirconia. The current collector layers are formed from a powder of silver particles having surface deposits of zirconium oxide to inhibit aging of the current collector layers. In order to provide contact between the oxygen containing feed stream and the electrolyte and to allow diffusion of the oxygen to and from the electrolyte, all of such current collectors and electrodes are porous. In the foregoing published patent application, the tube bundles are located within a heated enclosure having an inlet to receive the oxygen containing feed stream and an outlet in flow communication with manifolds connected to the tubular elements. Heating elements are provided within the insulation of the enclosure in order to heat the interior of the enclosure and the composite membrane elements to an operational temperature at which oxygen ion transport through the electrolyte can occur.
As known in the art, the oxygen produced by the electrically driven oxygen separation devices will be in proportion to the current drawn by the composite membrane elements. The composite membrane elements and specifically, the elements employed in such elements, however, degrade or age over time principally due to closure of the pores, evaporation of metallic elements such as silver, silver migration from the cathode to the electrolyte, reactions occurring between the electrodes and the electrolyte and delamination of the current collector. As such elements age, the resistance of the elements will increase and therefore, if a constant potential difference or voltage were applied, the current would decrease over time and therefore, the oxygen produced by the separation device. It is therefore, known that the power supply that is used to produce the potential difference to be applied should be a constant current power supply. However, if a constant current is applied, the voltage will steadily increase until failure of the membrane elements occurs. This failure will occur over a known time period and consequently, renewal of the device is scheduled in a conservative fashion to allow the composite membrane elements to be replaced in advance of any possible failure. This being said, it is very desirable to allow the electrically driven oxygen separation device to function as long as possible between renewal intervals due to interruption in service that necessarily occurs during renewal. Still another recognized way to control the electrical power to the separation zones is to measure the current and adjust the voltage so that power dissipated (product of voltage and current) in the separation zone is held constant. This aids control of the temperature of the separation device since dissipated power results in heating of the separation device, and if this is held constant, then the temperature of the separation device remains constant. However, this method has the disadvantage, that as the resistance increase over time, the current and oxygen flow will naturally decrease.
A further problem arises during startup of the separation device. The typical operating temperature for the elements used in the composite membrane elements is between 600° C. and 800° C. It is known that care must be taken in starting up the oxygen separation device in that the membrane elements also generate heat and if full power were applied to both the composite membrane elements and the heated enclosure during startup, overheating can occur leading to premature degradation if not failure of the composite membrane elements.
As will be discussed, among other advantageous features, the present invention provides a method and apparatus for controlling the application of the electrical power within an electrically driven oxygen separation device that allows the separation device to be operated for a longer time period than the prior art before replacement of the composite membrane elements becomes necessary and to allow the monitoring of the separation device to determine when replacement is to be performed.